QuestionI have a obd 2 scanner and always have random missfire codes. I've had my car in the shop where I bought it for repair with this problem about 7 times and it had been worse every time I get the car back. It runs ok untill the car warms up and then starts missing really bad and the engine light starts blinking non stop. The shop has told me they replaced the plugs and wires, cap and rotor button and the whole distributor. Also a crank sensor and feul pump. I'm lost at this point and I don't have anymore money to waste on parts and labor that won't fix the problem. Any input you could help with this would be great! Thank you!
AnswerHi Dan,
Because there are so many possible reasons for a code 300-306 and the shop has changed so many parts, I would focus on a couple of possibilities: The egr valve and the coolant temp sensor(because you say this happens after it warms up).
The first thing I would do is check the mechanical function of the egr valve which is located along a pipe that runs from the left exhaust manifold (on the side of the engine facing the front of the car) to the intake manifold area (where the incoming air in the large rubber tube enters the throttle). That valve is metal and has a round fitting with a vacuum hose attached, mounted on the top of the valve, and the round part is attached to the main body of the valve by a sort of saddle. Inside the open saddle you will see a metal rod (with a slot) that connects the round top fitting to to the valve body. That rod is a stem on the valve and it is supposed to go in and out in response to the engine vacuum which varies with RPM and throttle opening (gas pedal position). Take a spray can of WD-40 or other penetrating lubricant and spray the stem where it enters the valve body and then use a regular screwdriver blade tip inserted in the slot to lever the valve in and out to try and free up its motion. Also check all the rubber vacuum hoses associated with the valve and a nearby vacuum solenoid valve. If the valve seems to open (against the action of a spring) and close firmly by the spring action when using the screwdriver blade to facilitate the motion, then you should be able to now start the engine. You can, once it is idling, check that the valve stem moves when you rev the engine from idle to 2500 rpm and back to idle.
The coolant temp sensor is located at the thermostat housing where the top radiator hose connects to the engine. You can remove its electrical connector (lift tab on side to release) and measure the resistance across its terminal when the engine is cold (should be in the range of 5,000 ohms) and when the engine is fully warmed (should be 700-1,000 ohms). The wire colors of the sensor are tan/black and black/light blue. If the resistance doesn't get to below 1,000 ohms then you might want to replace it.
The way to routinely address multiple misfires is as follows:
The codes are based upon the detection of minor missing which causes subtle differences in the rpm as each cylinder fires to produce its rotational input (crankshaft speed sensor is the detector involved). The Chrysler troubleshooting manual lists the possible causes as:
secondary ignition wires, puel pump or fuel filter, injector harness connectors, ignition coil circuit, spark plugs, mechanical engine problem, contaminated fuel, water in fuel, PCM grounds, Injectors, Restricted exhaust, intake restriction, PCM, evap system, EGR system, Air gap at high rpm's, damaged sensor trigger wheel. That is quite a list!
The approach suggested is to check all electrical connectors and wiring, then do the tests in the following order:
secondary ignition
fuel delivery
engine vacuum
PCM power and ground connections
engine mechanical
The first one uses an engine analyzer scope to look at the high voltage pattern, then spray water on the cables to see if it changes. The possible repairs: individual spark plugs, coil or cable replacement
The second one involves pressure measurement and from there a variety of possible solutions
The third involves reading the intake manifold vacuum to see if it is steady at between 13 and 22 inches of HG. and if not the suspects depend upon the pattern shown by the gauge.
The PCM involves checking various wires at the PCM plugs
The last involves engine compression testing and a variety of other mechanical checks.
So I guess that beginning with the ignition wires makes sense unless you have a scope and the experience/knowledge at interpreting the patterns. You might try looking at the wires in the dark with the engine idling, then also spray some water mist on them to see in either case if you see arcing which would be a sign that the wires' insulation is breaking down. Then measure the coil resistances. If it passes that, then focus on the fuel filter and pump.
Roland
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